The role of protein phosphorylation on the sequence of events in stimulated human platelets is being studied. We have found pronounced changes in the phosphate content of six proteins (80K, 67K, 56K, 38K, 27K, and 20K daltons). The phosphate content of all proteins increases on stimulation except for the 67K phosphoprotein, which undergoes a rapid decrease in phosphate content. We then have studied the effect of various platelet stimulants and inhibitors to determine their effect on the phosphate content of each protein. This study has allowed us to propose a scheme for the location and function of each protein together with the role of calcium in a sequential array. Our next investigations will be to test our hypothesis by attempting to locate the individual proteins in subcellular fractions. The second investigation involves the role of prostaglandin synthesis in the secretion and aggregation stimulated platelets. We have found that it is possible to inhibit all prostaglandin synthesis without effecting the subsequent secretion and aggregation by thrombin stimulation in the cold on immobilized thrombin and then warming the platelets to 37 degrees C. Furthermore, prostaglandin synthesis does not occur at low doses of thrombin although secretion procedes to near completion. We tentatively propose that prostaglandin synthesis is not necessary for secretion or aggregation. We intend to study whether or not such a proposal is correct.